1. Field of the Invention
The present invention relates to a power tool such as an impact driver or an impact wrench used for fastening a screw or a bolt.
2. Description of the Related Art
Conventionally, a power tool used for fastening a screw or a bolt utilizing driving force of a motor has a function automatically stopping driving of the motor when a torque necessary for fastening the screw or the bolt reaches to a predetermined value due to the screw or the bolt is completely fastened.
In a first conventional power tool such as an impact wrench used for fastening a bolt, for example, shown in publication gazette of Japanese Patent Application 4-322974, a number of impact of a hammer is sensed and driving of a motor is automatically stopped when the number of impact reaches to a predetermined reference number.
In a second conventional power impact tool shown in publication gazette of Japanese Patent Application 9-285974, a rotation angle of a bolt is sensed and driving of a motor is stopped when the rotation angle reaches to a predetermined reference angle.
In a third conventional power impact tool shown in publication gazette of Japanese Patent Application 6-91551, an actual torque which is necessary for fastening a bolt is sensed and driving of a motor is stopped when the actual torque reaches to a predetermined reference value.
The first conventional power impact tool which stops the driving of the motor corresponding to the impact number and the second conventional power impact tool which stops the driving of the motor corresponding to the rotation angle respectively have a disadvantage that a large difference may occur between a desired torque and the actual torque for fastening the screw or the bolt. The difference causes loosening of the screw or the bolt due to insufficient torque when the actual torque is much smaller than the desired torque. Alternatively, the difference causes to damage the elements to be fastened by the screw or the bolt or to damage a head of the screw or the bolt due to superfluous torque when the actual torque is much larger than the desired torque.
On the other hand, the third conventional power impact tool which stops the driving of the motor corresponding to the actual torque for fastening the screw or the bolt needs a sensor provided on an output shaft for sensing the actual torque, so that it causes the cost increase and the upsizing of the power impact tool, even though the automatic stopping of the driving of the motor can be controlled precisely corresponding to the actual torque.
For solving the above-mentioned problems, in a fourth conventional power impact tool shown in publication gazette of Japanese Patent Application 2002-283248, a torque for fastening the bolt is estimated according to rotation speed of a shaft of a motor when an impact is applied to the bolt. It is judged that the bolt is completely fastened when the estimated torque is suddenly increased. The driving of the motor is stopped when the estimated torque is suddenly increased.
In the fourth conventional power impact tool, there is a possibility that the driving of the motor is stopped before the bolt is fastened completely, when the torque is temporarily increased due to trouble in fastening operation. In order to prevent the stop of the driving of the motor before the bolt is fastened completely, a reference torque for judging occurrence of trouble is defined, which is a little larger than the largest torque when the torque is temporarily increased due to the trouble. While the estimated torque has not been equal to or larger than the reference torque for judging occurrence of trouble, the driving of the motor is continued, so that the occurrence of malfunction that the driving of the motor is stopped before the bolt is fastened completely can be prevented.
As examples of cause of the trouble in the fastening operation, an irregularity of thread grooves of the bolt, a slight warp of a member interleaved between the bolt and the nut, decentering of the bolt or the nut, clipping of dust between the bolt and the nut, picking off of a surface coating of baking finish on the bolt, lifting of a member interleaved between the bolt and the nut, are recited.
In the fourth conventional power impact tool, it, however, is difficult to distinguish an instantaneous increase of the torque appeared in a variation of the torque due to the trouble in the fastening operation from the increase of the torque due to the bolt is fastened completely, since the variation of the torque while the bolt is fastened is irregular corresponding to combination of the bolt and the object to be fastened. Thus, there is a possibility that the driving of the motor is stopped before the bolt is fastened completely.